J.S.M. van Thiel de Vries

Observations and modeling of steep-beach grain-size variability

Novel observations of surface grain-size distributions are used in combination with intra-wave modeling to examine the processes responsible for the sorting of sediment grains on a relatively steep beach (slope?=?1:7.5). The field observations of the mean grain size collected with a digital camera system at consecutive low and high tides for a 2 week period show significant temporal and spatial variation. This variation is reproduced by the modeling approach when the surf zone flow-circulation is relatively weak, showing coarse grain sizes at the location of the shore break and finer sediment onshore and offshore of the shore break. The model results suggest that grain size sorting is dominated by the wave-breaking-related suspended sediment transport which removes finer sediment from the shore break and transports it both on-shore and offshore. The transport capacity of wave-breaking-related suspended sediment is controlled by the sediment response time scale in the advection-diffusion equation, where small (large) values promote onshore (offshore) transport. Comparisons with the observed beach profile evolution suggest a relatively short time scale for the suspended sediment response which could be explained by the vigorous breaking of the waves at the shore break.

Modeling the effect of wave‐vegetation interaction on wave setup

Aquatic vegetation in the coastal zone attenuates wave energy and reduces the risk of coastal hazards, e.g., flooding. Besides the attenuation of sea-swell waves, vegetation may also affect infragravity-band (IG) waves and wave setup. To date, knowledge on the effect of vegetation on IG waves and wave setup is lacking, while they are potentially important parameters for coastal risk assessment. In this study, the storm impact model XBeach is extended with formulations for attenuation of sea-swell and IG waves, and wave setup effects in two modes: the sea-swell wave phase-resolving (nonhydrostatic) and the phase-averaged (surfbeat) mode. In surfbeat mode, a wave shape model is implemented to capture the effect of nonlinear wave-vegetation interaction processes on wave setup. Both modeling modes are verified using data from two flume experiments with mimic vegetation and show good skill in computing the sea-swell and IG wave transformation, and wave setup. In surfbeat mode, the wave setup prediction greatly improves when using the wave shape model, while in nonhydrostatic mode (nonlinear) intrawave effects are directly accounted for. Subsequently, the model is used for a range of coastal geomorphological configurations by varying bed slope and vegetation extent. The results indicate that the effect of wave-vegetation interaction on wave setup may be relevant for a range of typical coastal geomorphological configurations (e.g., relatively steep to gentle slope coasts fronted by vegetation).

Measuring and modeling coastal dune development in the Netherlands

In the past couple of years, new coastal-dune research has sprung up in the Netherlands. In this paper, we give an overview of ongoing projects at Wageningen UR, Deltares, TU Delft and UTwente: how these are connected and what type of questions are addressed. There is an increasing demand for the understanding and prediction of coastal dune dynamics, both on the short (year) and long (100 years) term. We approach this from a variety of angles: scientific and applied, short-term and long-term, data-driven and model-based, biotic and abiotic, process-based and rule-based, and focused on components and integrated. We give examples of results and end with a discussion of the benefits of this integrated approach.

Morphological impact of a storm can be predicted three days ahead

People living behind coastal dunes depend on the strength and resilience of dunes for their safety. Forecasts of hydrodynamic conditions and morphological change on a timescale of several days can provide essential information to protect lives and property. In order for forecasts to protect they need be relevant, accurate, provide lead time, and information on confidence. Here we show how confident one can be in morphological predictions of several days ahead. The question is answered by assessing the forecast skill as a function of lead time. The study site in the town of Egmond, the Netherlands, where people depend on the dunes for their safety, is used because it is such a rich data source, with a history of forecasts, tide gauges and bathymetry measurements collected by video cameras. Even though the forecasts are on a local scale, the methods are generally applicable. It is shown that the intertidal beach volume change can be predicted up to three days ahead.

Rip Current Observations at Egmond aan Zee

Rip currents are narrow, seaward directed flows in the surf zone that can pose a serious threat to swimmers. This issue has received attention particularly on swell dominated coasts (such as the US, Australia, France and UK) where numerous field experiments have been undertaken. However, the threat of rip currents is less recognised on wind-sea dominated coasts such as the North Sea, even though a consistent number of swimmers drift offshore (in rip currents) and require rescue by surf lifeguards each year (for example at Egmond aan Zee, The Netherlands). In August 2011, a five day field experiment SEAREX (Swimmer Safety at Egmond – A Rip current Experiment) was conducted. Lagrangian velocity measurements were taken with drifter instruments and human drifters that were tracked via GPS. Three flow patterns were observed in the experiment: (1) a locally governed circulation cell, (2) a pattern in which the drifter initially floats offshore and then is advected by a strong tidal alongshore-directed current and (3) a meandering longshore current between the shoreline and the bar. A variety of rip current velocities were measured with the strongest being approximately 0.6 m/s. The field data was hindcasted with the numerical model XBeach. Based on this model the sensitivity of rip currents towards wave height, water level and rip channel depth was investigated. Both field and model data show that offshore velocities in a rip increase with increasing wave height and decreasing water level, but that the rip channel depth imposes an upper limit on the rip current velocity.

EFFECTS OF WAVE GROUPINESS ON DUNE EROSION

A research version of Delft 3D is used to simulate dune erosion by wave group forcing. The first three hours of a dune erosion experiment in the Delta flume are simulated and compared with measured data from this experiment. The effect of wave groupiness on dune erosion is studied by comparing model results including wave group forcing with model results using a time-invariant wave forcing.

MODELLING SCOUR IN FRONT OF DUNE REVETMENTS IN A SURF-BEAT MODEL

This paper presents adaptations to the XBeach model aimed at including the relevant processes for the generation of scour holes at the toe of a revetment. Dutch assessment rules for the safety of sea defenses need to be adjusted to cope with a combination of sandy dunes and hard elements. To that end, the XBeach model is prepared to be incorporated in the assessment rules. Until now, XBeach did not model scour hole development in front of dune revetments accurately. We suggest to include the advection of turbulence as well as the effect of backwash of short waves that creates additional turbulence in the model. Verification with three physical model experiments shows that with the suggested adaptations of the model a scour hole with significant depth can be modeled.